1. Field of the Invention
The present invention relates to a secondary battery. More particularly, this invention relates to a secondary battery having electrode assembly in which the capacity ratio between negative and positive active materials is reliably maintained to improve the battery performance and safety.
2. Description of the Prior Art
Generally, a secondary battery includes a roll electrode assembly having positive and negative electrodes applied with active materials and a separator disposed between the positive and negative electrodes; and a can in which the roll electrode assembly is inserted with the electrolyte. The secondary battery is a rechargeable battery using physical and chemical reactions between molecules of electrolyte and active materials. In recent years, the secondary battery has been widely used for portable products due to its compact in size and large capacity.
The secondary batteries are classified into a nickel-cadmium, nickel-metal hydride, and lithium ion batteries depend on materials used as the positive and negative electrodes or the electrolyte, and are further classified according to their shape into cylindrical, package and prism types.
Especially, the prism type secondary battery comprises a roll electrode assembly consisting of a positive electrode, a negative electrode, and a separator disposed between the positive and negative electrodes; a can in which the electrode assembly is inserted with the negative electrode contacted therewith; and a cap assembly coupled to the positive electrode of the electrode assembly. Internal insulating plates are provided at upper and lower ends of the electrode assembly to prevent the electrode assembly from contacting the cap assembly and the can. The cap assembly is provided with a negative plate which is welded to the upper end of the can, a positive plate formed on a central portion of the cap assembly, and an external insulating plate disposed between the negative and positive plates. The positive plate contacts the positive electrode of the electrode assembly by a tap and supported by a rivet passing through the negative and positive plates. An insulating gasket is disposed between the rivet and the negative plate.
In the above described conventional prism type secondary battery, the positive and negative electrodes (hereafter referred as electrode) are manufactured through multiple steps. That is, active material is first applied on opposite surfaces of an electrode substrate at a predetermined thickness. Then, the electrode substrate applied with the active material is dried and pressed, thereby obtaining the electrode. The active material allows the absorption and release of the electrons or ions to generate or retain electromotive force.
In the secondary battery, the capacity ratio of the negative active material with respect to the positive active material ("N/P ratio") is usually at about 1.2 to 1.4 in order to obtain a reserve by which positive ions generated from the positive electrode can be absorbed into the negative electrode during charge and discharge. For example, if the N/P ratio is less than 1, a metal oxide is extracted or the electrolyte leaks. This results in the deterioration of the charge and discharge ability of the secondary battery, and even in the explosion of the secondary battery by the increase of the internal pressure of the battery.
Referring to FIG. 6, there is shown a sectional view of a conventional roll electrode assembly 100. The roll electrode assembly 100 is substantially oval having straight parts St and curve parts L. The roll electrode assembly comprises a positive electrode 100b applied with a positive active material, a negative electrode applied with a negative active material, and a separator 102 disposed between the positive and the negative electrodes 100b and 100a.
At the straight parts St of the assembly 100, the capacity of the positive active material of the positive electrode 100b is the same as that of the negative active material of the negative electrode 100a as the length of the positive electrode 100b is equal to that of the negative electrode 100a. However, at the curve parts L of the assembly 100, as shown in FIG. 7, the capacity of the positive active material of the positive electrode 100b is different from that of the negative active material of the negative electrode 100a as the length of the positive electrode 100b differs from that of the negative electrode 100a.
Describing more in detail with reference to FIGS. 7 and 8, each circumference length S.sub.n of the positive and negative electrodes at the curve parts is as follows: EQU S.sub.n =.theta.*R.sub.n
Where, .theta. is an angle between a first line radially extending from a center point of the curved portion and a second line radially extending from the center point; and R.sub.n is a straight distance from the center point to each electrode (n indicates the turn order of the positive and negative electrodes).
Therefore the difference of the circumference length between two adjacent electrodes becomes as follows: EQU S.sub.n -S.sub.n-1 =(.theta.*R.sub.n)-(.theta.*R.sub.n-1)
In addition, when the thickness of the positive and negative active materials of the two adjacent electrodes is T, since T is equal to a distance between the adjacent electrodes, the difference of circumference length of two adjacent electrodes becomes as follows: EQU S.sub.n -S.sub.n-1 =(.theta.*R.sub.n)-(.theta.*R.sub.n-1)=.theta.(R.sub.n-1 +T)-.theta.R.sub.n-1 =.theta.T
And, the difference of the capacity of the two adjacent electrodes becomes as follows:
.theta.T*T* the vertical axis length of the electrode
Accordingly, in the conventional secondary battery, as shown in FIG. 9(A), when the negative electrode is located at the outermost of the roll electrode assembly, the N/P ratio will be maintained more than 1.
However, as shown in FIG. 9(B), when the positive electrode is located at the outermost of the roll electrode assembly, there may be a zone where the N/P ratio becomes less than 1. This zone increases the potential for an extraction of a metal oxide during the charge and discharge of the prism type secondary battery, resulting in the deterioration of the performance of the battery. Especially, when the battery is charged and discharged at a high ratio, this causes the deterioration of the battery safety.